Although the audio status is set in onTrackChanged for media groups, and
the function is called when the media groups are first set up, the track
is not always considered changed. This means that for demuxed audio, the
main loader may still think it should be using its own audio itself,
leading to issues when crossing discontinuities (i.e., the main loader
will cross the discontinuity before waiting for the audio loader to be
ready, leading to audio timestamps that aren't correct).

This change ensures that the audio status is set on setup, regardless of
whether the track is considered changed. Subsequent changes are handled
in onTrackChanged.